Emigration and Density Dependence in Yellowstone Bison

Abstract

: Understanding the relative importance of density‐dependent and density‐independent feedback on population growth is essential for developing management strategies to conserve wildlife. We examined a 99‐year time series of annual counts and removals for 2 bison (Bison bison) herds occupying northern and central Yellowstone National Park in the western United States. Yellowstone's aggressive management intervention effectively recovered bison from 46 animals in 1902 to > 1,500 animals in 1954. Supplemental feeding of the northern herd facilitated rapid growth (r = 0.16) during 1902 to 1952. Augmentation of the central herd with 71 animals also led to rapid growth over 1936 to 1954 (r = 0.10). In 1969, manipulative management ceased in the park, and we detected evidence of density‐dependent changes in population growth rates for both herds during 1970 to 2000 as numbers increased to >3,000 animals. The central herd showed evidence of a constant density‐dependent response over 1970 to 2000. In contrast, density dependence had a stronger effect on the northern herd's growth rate during 1970 to 1981 than during 1982 to 2000. We found evidence to suggest that these trends resulted from pulses of emigration from the central herd to the northern range beginning in 1982 in response to resource limitation generated by an interaction between density and severe snow pack. Corroborative evidence supporting this interpretation included 1) the annual growth of the central herd was negatively correlated with snow pack but that of the northern herd was not, 2) growth rates of the central and northern herds were uncorrelated during 1970 to 1981 but significantly and negatively correlated during 1982 to 2000, and 3) the northern herd could not have sustained the high removals experienced during 1984 to 2000 without immigration. Density‐related emigration from the central herd to the northern range may be fueling bison emigration onto private and public lands where large‐scale removals occur, exacerbating the brucellosis controversy for natural resource managers.

State‐space modeling to support management of brucellosis in the Yellowstone bison population

“…Earlier empirical work found nominal support for an effect of density on recruitment (Fuller et al 2007a). We estimated separate, time-invariant fecundities for seronegative animals ( f n ), animals that have seroconverted and are actively infectious ( f c ), and recovered animals that are seropositive ( f p ); see Fig.…”

“…The model omits covariates describing weather conditions, e.g., drought severity, which have been included in other models of bison population dynamics in Yellowstone (Fuller et al 2007a). We justify this omission because our central objective was to develop a forecasting model.…”

“…The model of Fuller et al (2007a) for bison fecundity includes the Palmer Drought f g s ði;tÞ n ði;tÀ1Þ þ P i2 2;3;5;6;7 f g s ði;tÞ n ði;tÀ1Þ…”

Disentangling the effects of climate, density dependence, and harvest on an iconic large herbivore's population dynamics

“…Controlled hunting and translocation (collectively, extraction) are used to manage bison numbers in and between areas within metapopulation management programs. Managers must also be mindful of densitydependent competition for available forage because of feedback effects on bison dispersal and demography, as well as the potential to create conflict with the cattle industry (Fuller et al 2007a, b, Plumb et al 2009, Koons et al 2012. Understanding the influences of perturbations to extraction rates, climate variables, and strength of density dependence on population dynamics will thus be especially critical for managing bison amid the significant pressures imposed by land use, water use, and climate change (Lemly et al 2000, Pringle 2000, Northrup and Wittemyer 2013.…”

Effects of bison on willow and cottonwood in northern Yellowstone National Park

“…Meanwhile, bison on the northern range increased from 455 in the summer of 1997 (following the removal of 725 the previous winter), to 2070 bison in 2007, the highest count on the northern range in the history of the park (Meagher, 1973;White et al, 2011). Since 1984 bison have congregated in the Lamar Valley in summer as well as winter, and some bison have moved from central Yellowstone to the northern range (Taper et al, 2000;Gates et al, 2005;Fuller et al, 2007). Ripple et al (2010) hypothesized that the bison increase on the northern range may be part of a secondary trophic cascade, where wolves reduced elk density, thereby releasing bison from interspecific competition, resulting in higher bison densities and greater effects from bison on forage plants.…”